Polymer electrolyte fuel cells have high energy conversion efficiency, is clean, is quiet, and an operation temperature thereof is 100° C. or less. Therefore, the polymer electrolyte fuel cell is expected as a future energy generation device.
Further, because the polymer electrolyte fuel cell has high energy density and the low operation temperature, in recent years, not only application to automobiles, household power generators, or the like, but also application to portable electrical apparatuses such as mobile phones, notebook personal computers, or digital cameras are taken in consideration. The polymer electrolyte fuel cell is capable of driving the portable apparatuses for a long time compared to a related-art lithium ion secondary battery, thereby receiving attention.
The polymer electrolyte fuel cell generally has a structure in which a membrane electrode assembly (hereinafter, abbreviated as “MEA” in some cases) is sandwiched between conductive separators. The MEA has a structure in which a polymer electrolyte membrane is sandwiched between a pair of porous gas diffusion electrodes.
In general, the polymer electrolyte fuel cell has a problem in that, with a passage of power generation time, a voltage is gradually reduced due to a flooding phenomenon, and the power generation stops at last.
In the flooding phenomenon, because water produced in a reaction remains in spaces of the gas diffusion electrode, supply of a fuel gas serving as a reactant is inhibited, thereby causing voltage reduction or stoppage of the power generation reaction. In particular, the flooding tends to occur in the gas diffusion electrode on the cathode side in which water is produced. Accordingly, enhancement of dissipativeness of the product water from the gas diffusion electrode is an important factor affecting performance stability of the fuel cell.
In order to suppress the flooding phenomenon, the gas diffusion electrode is usually added with a hydrophobic resin such as polytetrafluoroethylene (PTFE), thereby making inside of pores of the gas diffusion electrode hydrophobic.
Further, in the fuel cell for application to automobiles and household generators, there is used an auxiliary device such as a blower or a temperature/humidity control system to maintain the gas diffusion electrodes under an appropriate temperature/humidity environment.
However, in the fuel cell for the application to portable electrical apparatuses, in order to suppress electrical power consumption of the auxiliary devices and battery dimensions, it is demanded that a minimum number of auxiliary devices be used to achieve downsizing and weight reduction of the fuel cell.
Thus, in the fuel cell for the portable electrical apparatuses, it is desirable that the temperature/humidity control system be not used, and it is preferable that a fan or a blower be not used or be used in a minimum airflow rate when used. The gas diffusion electrode of the fuel cell for the portable apparatuses is required of not causing flooding even when the airflow rate is low or zero.
Thus, in the gas diffusion electrode of the fuel cell for the portable electrical apparatuses, there is demanded a function capable of coping with the flooding even when the airflow rate is low.
An example of the gas diffusion electrode includes one in which a catalyst is carried by a carbon fiber sheet (for example, Japanese Patent Application Laid-Open No. H08-106915, Japanese Patent Application Laid-Open No. H11-510311, Japanese Patent Application Laid-Open No. 2002-534773, Japanese Patent Application Laid-Open No. 2006-032170, Japanese Patent No. 3773325, and Japanese Patent No. 3444530). As the carbon fiber sheet, carbon paper or carbon cloth is used most widely, which is formed of carbon fibers each having a diameter of about several micrometers. In order to rapidly discharge the product water caused by the fuel cell reaction, the carbon fibers are generally coated with a hydrophobic resin such as PTFE, thereby being hydrophobically treated.
However, in a case of the above-mentioned structure, an interface between the gas diffusion electrode and the separator is fibrous, so a peak pore diameter in the interface is several tens of micrometers or more, which is too large. Thus, there is a problem in that capillary forces of the pores are weak and sufficient drainage property cannot be obtained, thereby easily causing flooding.
In order to solve the problem, what has been the mainstream recently is a gas diffusion electrode obtained by laminating three layers, which are (1) a carbon fiber layer formed of carbon fibers, (2) a carbon fine particle layer, and (3) platinum carrying carbon fine particles (for example, Japanese Patent No. 3773325, Japanese Patent No. 3444530, Japanese Patent No. 3594533, or ELAT (US registered trademark, E-TEK)).
Currently, (1) the carbon fiber layer and (2) the carbon fine particle layer are laminated to each other to be commercially available as a gas diffusion layer from plural makers. Accordingly, in this specification, this is referred to as the gas diffusion layer (hereinafter, abbreviated as “GDL” in some cases) (commercial example: trade name LT-1200W (E-TEK), Avcarb 2120 (Ballard Power Systems Inc.), CARBEL (registered trademark, JAPAN GORE-TEX INC)).
In the GDL, the interface between the carbon fiber layer and the carbon fine particle layer is not necessarily definite. That is, in many cases, and the carbon fine particle layer partially intrudes into the carbon fiber layer, or, carbon fine particles having the same composition as that of the carbon fine particle layer are also arranged between the carbon fibers.
In general, the carbon fine particle layer is formed of an aggregate of the carbon fine particles, which includes the hydrophobic resin such as PTFE as a binder. For the carbon fine particles, there may be often used acetylene black which is graphitized.
Pore diameters of the carbon fine particle layer are in a distribution of about 0.1 to several micrometers. Accordingly, the carbon fine particle layer has a low water permeability and functions to inhibit the draining from the catalyst layer to some extent, thereby achieving an effect of retaining moisture of the electrolyte in the MEA. Further, the carbon fine particle layer also has an effect of preventing the product water remaining in the fiber layer from flowing back to the catalyst layer.
As described above, by causing the gas diffusion electrode to have a three-layer structure, even when the product water remains in a part of the GDL, air supply to the catalyst layer is maintained, so the flooding can be prevented.
However, according to Japanese Patent Application Laid-Open No. 2002-343369, there is pointed out a problem in that, since the interface between the GDL and the separator is fibrous, and the interface is a hydrophilic/hydrophobic interface, the product water tends to remain in the interface, thereby easily causing the flooding.
Japanese Patent Application Laid-Open No. 2002-343369 discloses that, with the provision of a layer formed of the same composition as that of the carbon fine particle layer to the interface, the above-mentioned problem can be solved. Further, U.S. Pat. No. 6,605,381 also discloses a gas diffusion electrode of the same structure.
On the other hand, Japanese Patent Application Laid-Open No. 2006-049278 and Japanese Patent Application Laid-open No. 2001-519594 each discloses a method of forming a catalyst layer thinner than that of the related art by using a vapor phase deposition method such as a sputtering method or an ion plating method.
As described above, recently, there has been developed a method of forming a catalyst layer in a form of a thin film by using a manufacturing method such as the sputtering method. The thin catalyst layer has a membrane thickness smaller than that of the related art, so a diffusion distance of a reactant gas, the product water, or the like is short and material diffusibility thereof is superior. Accordingly, there is an advantage in that high power generation property can be obtained even with a small amount of catalyst metal.
Further, an organic solvent amount required at a time of manufacture can be saved compared to that in the related art, so there is an advantage in that an environmental load is small.
However, the thin film catalyst layer formed by using the sputtering method illustrated in each of Japanese Patent Application Laid-Open No. 2006-049278 and Japanese Patent Application Laid-open No. 2001-519594 does not employ a carrier, so a pore volume is reduced compared to that in the related-art platinum carrying carbon catalyst layer.
Therefore, there is a problem in that, even when the product water remaining in the catalyst layer is small, the flooding easily occurs. In a case where the thin film catalyst layer and the GDL having the structure as described in Japanese Patent No. 3773325, Japanese Patent No. 3444530, Japanese Patent No. 3594533, Japanese Patent Application Laid-Open No. 2002-343369, and U.S. Pat. No. 6,605,381 are combined with each other, the draining from the catalyst layer is inhibited by the carbon fine particle layer, so there is a problem in that the thin film catalyst layer easily causes the flooding.
Further, in a case where the thin film catalyst layer is applied to the gas diffusion electrode having a structure according to Japanese Patent Application Laid-Open No. H08-106915, Japanese Patent Application Laid-Open No. H11-510311, or Japanese Patent Application Laid-Open No. 2002-534773, mentioned above, the capillary force is weak as described above. Accordingly, sufficient draining property cannot be obtained, so, again, there is a problem in that the flooding easily occurs.
In view of this, there is a demand for a technology for suppressing the flooding of the thin film catalyst layer formed by the sputtering method and the ion plating method, the technology involving low manufacturing costs.